Paper or synthetic paper on which printing is performed has previously been used as labels stuck on containers such as plastic bottles for indicating their contents.
For the labels using such paper, however, top surface printing can not but be performed because of opacity. As a result, the labels have the disadvantage that prints are stained or disappear by the spillover of the contents, or that the labels are broken thereby. There is further the problem that the labels must be formed to a thickness of more than 100 microns due to their low rigidity.
In contrast, the use of plastic labels on the plastic bottles provides the advantages that the labels are unnecessary to be stripped in recovering and recycling the bottles, and that the labels are resistant to water wetting.
If the bottles are formed of polyethylene and the labels are also formed of polyethylene, the above-described advantages can be more utilized because both of the bottles and the labels are formed of the same material.
On the other hand, blow molding is used for forming bottles. Blow molding is the method of forming a hollow article by air blowing, which comprises plasticizing a thermoplastic resin in an extruder, extruding the plasticized resin to form a parison, enclosing the parison not cooled to solidification yet in a mold, blowing air in the inside of the parison to expand it, and cooling the parison while pressing it to the inner wall of the mold.
On such blow molding, if a label is previously set in the mold and can be stuck on the surface of a bottle concurrently with blow molding, laborious labeling operations can be omitted.
However, the labels for this purpose (labels for in-molding) are stuck on the curved surfaces of the bottles, so that they are required to maintain their form as the labels, namely to have formability, and also required to be excellent in strength, clarity and rigidity.
There has been previously provided a polyethylene label comprising an oriented polyethylene film in which the degree of crosslinking inwardly decreases across the thickness of the film, printing being performed on one surface thereof and an adhesive layer being formed on the printed surface (Japanese Utility Model Application No. 63-116860/1988).
This label is good in clarity, and back surface printing can be performed thereon. The label can provide an integral feeling and a high-class feeling as if curved-surface printing is performed on a cylindrical container itself. Further, the label is sufficient in rigidity and strength, and therefore it is possible to be thinly formed. Furthermore, the label is resistant to water wetting and contamination, and unnecessary to be stripped in recovering and recycling the polyethylene bottle. Accordingly, the label is suitable for use as a label for in-molding which can be stuck on the bottle concurrently with blow molding of the bottle.
However, the intensive studies of the present inventors have revealed that a blister phenomenon is liable to take place due to the difficulty of complete degassing for a sealant for the label, when the label is stuck on the surface of the bottle concurrently with blow molding. This phenomenon is particularly remarkable when the molding is carried out by extrusion coating using low-density polyethylene.
The intensive studies of the present inventors have further revealed that the blister phenomenon is liable to take place due to the difficulty of complete degassing also for a sealant applied by the hot lacquer method of applying a hot lacquer and drying it, when the label is stuck on the surface of the bottle concurrently with blow molding.
Such a blister phenomenon seems temporarily unobserved, but sometimes becomes remarkable later due to the existence of very fine bubbles.
It is therefore a primary object of the present invention to provide a clear polyethylene label suitable for use as a label for in-molding, on which back surface printing can be performed, which can provide an integral feeling and a high-class feeling as if curved-surface printing is performed on a cylindrical container itself, prints on which do not disappear or which is not broken by the spillover of the contents, which is sufficient in rigidity and strength and therefore is possible to be thinly formed, and which is resistant to water wetting and contamination, which is unnecessary to be stripped in recovering and recycling a polyethylene bottle when the label is stuck thereon, and can be stuck on the bottle concurrently with blow molding of the bottle, as with the above label previously proposed.
Another object of the present invention is to provide a label in which the blister phenomenon does not occur when the label is stuck on the surface of a bottle concurrently with blow molding of the bottle.
A further object of the present invention is to improve the label previously proposed and to provide a polyethylene label excellent in adhesive strength between printing ink and an adhesive when the adhesive layer is formed on a printed surface thereof.